test $# = 1||
   exec echo usage: $0 itag-no

   sed -n '
   s/http:/https:/;
   s/m.youtube/www.youtube/;
   s/youtu.be/youtube/;
   s/embed./watch?v=/;
   #etc.;
   s/;.*//;
   w.'"$0"''
   read a  .$0b;
   rm .$0a;

   read b